Change the "malloc" test main for Cortex-A
Change the "malloc" test main for Cortex-A

"malloc" test is used a simple thread that executes only malloc and free, but it seems that there is a difference in the stack used between Cortex-A and Cortex-M.
As a result of check, in Cortex-A, program aborts because of the lack of stack  when THREAD_STACK_SIZE is 256, workes properly when THREAD_STACK_SIZE is 512 at least. Since it seems that the definition processing of "DEFAULT_STACK_SIZE" was gone from Mbed OS by updating CMSIS5/RTX5, I setted 512 directly for Cortex-A.

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